The Internet Domain Name System (DNS) consists of the syntax to specify the names of entities in the Internet in a hierarchical manner, the rules used for delegating authority over names, and the system implementation that actually maps names to Internet addresses. DNS data is maintained in a group of distributed hierarchical databases.
1.1. Scope of Document
The Berkeley Internet Name Domain (BIND) implements a domain name server for a number of operating systems. This document provides basic information about the installation and care of the Internet Systems Consortium (ISC) BIND version 9 software package for system administrators.
This manual covers BIND version 9.16.33.
1.2. Organization of This Document
In this document, Chapter 1 introduces the basic DNS and BIND concepts. Chapter 2 describes resource requirements for running BIND in various environments. Information in Chapter 3 is task-oriented in its presentation and is organized functionally, to aid in the process of installing the BIND 9 software. The task-oriented section is followed by Chapter 4, which is organized as a reference manual to aid in the ongoing maintenance of the software. Chapter 5 contains more advanced concepts that the system administrator may need for implementing certain options. Chapter 6 addresses security considerations, and Chapter 7 contains troubleshooting help. The main body of the document is followed by several appendices which contain useful reference information, such as a bibliography and historic information related to BIND and the Domain Name System.
1.3. Conventions Used in This Document
In this document, we generally use
Fixed Width text to indicate the
following types of information:
mailing list names
new terms or concepts
literal user input
Text in “quotes,” bold, or italics is also used for emphasis or clarity.
1.4. The Domain Name System (DNS)
This document explains the installation and upkeep of the BIND (Berkeley Internet Name Domain) software package. We begin by reviewing the fundamentals of the Domain Name System (DNS) as they relate to BIND.
1.4.1. DNS Fundamentals
The Domain Name System (DNS) is a hierarchical, distributed database. It stores information for mapping Internet host names to IP addresses and vice versa, mail routing information, and other data used by Internet applications.
Clients look up information in the DNS by calling a resolver library,
which sends queries to one or more name servers and interprets the
responses. The BIND 9 software distribution contains a name server,
named, and a set of associated tools.
1.4.2. Domains and Domain Names
The data stored in the DNS is identified by domain names that are organized as a tree according to organizational or administrative boundaries. Each node of the tree, called a domain, is given a label. The domain name of the node is the concatenation of all the labels on the path from the node to the root node. This is represented in written form as a string of labels listed from right to left and separated by dots. A label need only be unique within its parent domain.
For example, a domain name for a host at the company Example, Inc.
com is the top-level domain
example is a subdomain of
ourhost is the name of the host.
For administrative purposes, the name space is partitioned into areas called zones, each starting at a node and extending down to the “leaf” nodes or to nodes where other zones start. The data for each zone is stored in a name server, which answers queries about the zone using the DNS protocol.
The data associated with each domain name is stored in the form of resource records (RRs). Some of the supported resource record types are described in Types of Resource Records and When to Use Them.
For more detailed information about the design of the DNS and the DNS protocol, please refer to the standards documents listed in Requests for Comment (RFCs).
To properly operate a name server, it is important to understand the difference between a zone and a domain.
As stated previously, a zone is a point of delegation in the DNS tree. A zone consists of those contiguous parts of the domain tree for which a name server has complete information and over which it has authority. It contains all domain names from a certain point downward in the domain tree except those which are delegated to other zones. A delegation point is marked by one or more NS records in the parent zone, which should be matched by equivalent NS records at the root of the delegated zone.
For instance, consider the
example.com domain, which includes names
example.com zone includes only delegations for the
bbb.example.com zones. A zone can map
exactly to a single domain, but could also include only part of a
domain, the rest of which could be delegated to other name servers.
Every name in the DNS tree is a domain, even if it is terminal, that
is, has no subdomains. Every subdomain is a domain and every domain
except the root is also a subdomain. The terminology is not intuitive
and we suggest reading RFC 1033, RFC 1034, and RFC 1035 to gain a complete
understanding of this difficult and subtle topic.
Though BIND 9 is called a “domain name server,” it deals primarily in
terms of zones. The
secondary declarations in the
file specify zones, not domains. When BIND asks some other site if it is
willing to be a secondary server for a domain, it is actually asking
for secondary service for some collection of zones.
1.4.5. Caching Name Servers
The resolver libraries provided by most operating systems are stub resolvers, meaning that they are not capable of performing the full DNS resolution process by themselves by talking directly to the authoritative servers. Instead, they rely on a local name server to perform the resolution on their behalf. Such a server is called a recursive name server; it performs recursive lookups for local clients.
To improve performance, recursive servers cache the results of the lookups they perform. Since the processes of recursion and caching are intimately connected, the terms recursive server and caching server are often used synonymously.
The length of time for which a record may be retained in the cache of a caching name server is controlled by the Time-To-Live (TTL) field associated with each resource record.
Even a caching name server does not necessarily perform the complete recursive lookup itself. Instead, it can forward some or all of the queries that it cannot satisfy from its cache to another caching name server, commonly referred to as a forwarder.
Forwarders are typically used when an administrator does not wish for all the servers at a given site to interact directly with the rest of the Internet. For example, a common scenario is when multiple internal DNS servers are behind an Internet firewall. Servers behind the firewall forward their requests to the server with external access, which queries Internet DNS servers on the internal servers’ behalf.
Another scenario (largely now superseded by Response Policy Zones) is to send queries first to a custom server for RBL processing before forwarding them to the wider Internet.
There may be one or more forwarders in a given setup. The order in which
the forwarders are listed in
named.conf does not determine the
sequence in which they are queried; rather,
named uses the response
times from previous queries to select the server that is likely to
respond the most quickly. A server that has not yet been queried is
given an initial small random response time to ensure that it is tried
at least once. Dynamic adjustment of the recorded response times ensures
that all forwarders are queried, even those with slower response times.
This permits changes in behavior based on server responsiveness.
1.4.6. Name Servers in Multiple Roles
The BIND name server can simultaneously act as a primary for some zones, a secondary for other zones, and as a caching (recursive) server for a set of local clients.
However, since the functions of authoritative name service and caching/recursive name service are logically separate, it is often advantageous to run them on separate server machines. A server that only provides authoritative name service (an authoritative-only server) can run with recursion disabled, improving reliability and security. A server that is not authoritative for any zones and only provides recursive service to local clients (a caching-only server) does not need to be reachable from the Internet at large and can be placed inside a firewall.